The present invention relates to a light amplifying device employing a semiconductor optical amplifier. More particularly, the present invention relates to a light amplifying device equipped with functions (ALC: Automatic Level Control, APC: Automatic Power Control), which control the level/power of output signal light to a constant value, and which can be used in a wavelength division multiplexed communication system.
Recently, development of a wavelength division multiplexed communication system, which will realize transmission through an optical fiber cable by multiplexing plural signal light of different wavelengths, has advanced to meet the increasing demands on communication systems. In a wavelength division multiplexed communication system, many optical components are used for light combination and light division, therefore, a light signal is attenuated by the losses of each optical component. To compensate for such losses, a light amplifying device is used.
Compared to the conventional optical fiber communication system, the wavelength division multiplexed communication system needs many more light amplifying devices, therefore, a light amplifying device must be compact and have low power consumption. Moreover, such a light amplifying device needs to have a large dynamic range to handle large variations in power level of the input signal light and functions (ALC/APC functions) to control the level/power of the output signal light to a constant value. Among various types of light amplifying devices, a semiconductor optical amplifier (SOA) is compact and has a low power consumption and, therefore, attracts interest as a light amplifying device to compensate for losses in the wavelength division multiplexed communication system.
To realize the ALC function in a light amplifying device employing a semiconductor optical amplifier, a method may be used in which electric current supplied to the semiconductor optical amplifier is changed to vary the gain of the semiconductor optical amplifier, but this method will bring about a problem in that the saturation level of the output light varies if the electric current supplied to the semiconductor optical amplifier is changed, and distortion of the signal occurs due to the pattern effect. Therefore, a structure is employed, in which the semiconductor optical amplifier is driven under a fixed condition almost near the upper limit so that the amplification factor is maintained constant, an attenuator to attenuate light input to the semiconductor optical amplifier is provided, and light is input to the semiconductor optical amplifier, after being attenuated by the attenuator, so as to have a constant power.
FIG. 1 is a diagram that shows the structure of an example conventional light amplifying device that employs a semiconductor optical amplifier and has an ALC function. As shown schematically, the output light of a modulator integrated DFB laser diode (MI-DFB-LD) 11 is modulated by the signal from a modulation signal source 12. The light output from the MI-DFB-LD 11 is amplified to a fixed power by a light amplifying device 21.
The optical amplifying device 21 comprises an attenuator (Att) 23, which attenuates the input light and the attenuation factor of which can be changed, a divider 24, which divides the output of the attenuator 23 in the ratio, for example, of 10:1, a power meter 25, which detects the power of the light of lower strength divided by the divider 24, a control unit 26, which controls the attenuation of the attenuator 23 according to the light detected by the power meter 25, and a semiconductor optical amplifier (SOA) 22, which amplifies the light of larger strength divided by the divider 24. The SOA 22 is driven under a fixed condition.
Next the ALC operation in the light amplifying device in FIG. 1 is described with reference to FIG. 2.
When modulated signal light is amplified in the SOA 22, it is necessary to set the average light output power of the SOA 22 lower than the saturated light output by a few dB in order to avoid signal distortion due to the pattern effect based on the gain saturation, in which the output light is saturated. Here, for example, it is set 5 dB lower than the saturated light output PS (dBm). Therefore, the permissible maximum value PSM (dBm) of the average light output power of the SOA 22 is shown as follows.
PSM(dBm)=PS(dBm)xe2x88x925dBxe2x80x83xe2x80x83(1)
When the level of the output signal light of the light amplifying device is controlled to be constant by the ALC function, it is desirable that the target level is as large as possible, therefore, the target level is set to PSM (dBm).
As described above, the drive condition of the SOA 22 is fixed and, because the input light is amplified with a fixed gain Gs (dB), the permissible maximum value PSIM (dBm) of the average light input power to the SOA 22 is show as follows.
PSIM(dBm)=PSM(dBm)xe2x88x92Gs(dB)xe2x80x83xe2x80x83(2)
Therefore, if the average light input power PSIM (dBm) to the SOA 22 is constantly adjusted so as to be PSIM (dBm) by the variable attenuator 23, the level of the output signal light of the SOA 22 is constantly a fixed PSM (dBm).
The lower limit PIMIN (dBm) of the average light input power to the light amplifying device 21, when no attenuation is carried out by the attenuator 23, is shown by the following expression, the basic loss LA1 of the attenuator 23 and the loss LD1 of the optical divider 24 being taken into account.
PIMIN (dBm)=PSIM(dBm)+LA1+LD1xe2x80x83xe2x80x83(3)
The dynamic range xcex94PIN (dB) of the light amplifying device 21 is determined by the maximum quantity of attenuation LATM (dB) of the attenuator 23.
xcex94PIN(dB)=LATM(dB)xe2x80x83xe2x80x83(4)
Therefore, the upper limit PIMAX (dBm) of the average light input power of the light amplifying device 21 is determined by the following expression.
PIMAX (dBm)=PIMIN (dBm)+xcex94PIN(dB) =PSIM(dBm)+LA1+LD1+LATM(dB)xe2x80x83xe2x80x83(5)
From the standpoint of generality, it is preferable that a light amplifying device can be used commonly for signal light of various wavelengths. For example, when combining plural types of signal light of different wavelengths transmitted from a transmitter, after each signal light is amplified to a fixed value, respectively, or when recombining plural types of signal light of different wavelengths received by a relay device, after each signal light is divided and amplified to a fixed value, individually, it is troublesome to use plural different light amplifying devices according to each wavelength, or to set different conditions even if a single light amplifying device is used.
The light amplifying device shown in FIG. 1 can provide light output of a fixed level as long as the wavelength of the signal light is fixed. Because the gain of the SOA 22 has wavelength dependence, however, levels of the output signal light of the light amplifying device vary depending on the wavelength of signal light, even though the average light input power to the SOA21 is controlled to be constant in the structure in FIG. 1. Therefore, in the structure in FIG. 1, if only the average light input power of the signal light is monitored and the wavelength of the signal light is not monitored, the average light output power does not remain constant, and the average light output power of the signal light varies depending on the wavelength by the difference xcex94Gs (dB) between the maximum gain GsH (dB) and the minimum gain GsL (dB) of the SOA 22 in the range of the used wavelength, as shown below.
xcex94Gs(dB)=GsH(dB)xe2x88x92GsL(dB)xe2x80x83xe2x80x83(6)
In order to keep the average light output power constant even when the wavelength varies, a mechanism is needed, which detects the output of the SOA and attenuates the output of the SOA according to the detected value. FIG. 3 shows an example of a structure, in which mechanisms that detect the level of the signal light and attenuate according to the detected value are provided on both sides of the SOA in order to obtain a fixed average light output power regardless of the wavelength.
In the structure shown in FIG. 3, a selector 20, which selects a light signal to amplify from among plural light signals of different wavelengths, is provided and the light signal selected by the selector 20 is input into the light amplifying device 21. Therefore, only one light signal is input into the light amplifying device 21 at one time, but the input light signals have plural wavelengths.
In the light amplifying device 21, a similar attenuation mechanism is provided for the output of SOA 22, in addition to the structure in FIG. 1. As shown schematically, this attenuation mechanism comprises a second attenuator (Att) 27, which attenuates the output of the SOA 22 and the attenuation factor of which can be changed, a second divider 28, which divides the output of the second attenuator 27, a second power meter 29 that detects the power of the light divided by the second divider 28, and a second control unit 30 that controls the quantity of attenuation of the second attenuator 27 according to the light detected by the second power meter 29.
The ALC operation of the light amplifying device shown in FIG. 3 is described with reference to FIG. 4.
It is assumed that the saturated light output of the SOA 22 slightly depends on wavelength and the saturated light output is constant at PS in the assumed range of wavelengths. As described above, if the average light output power of the SOA 22 is lower than the saturated light output PS by 5 dB, the permissible maximum value PSM (dBm) of the average light power of the SOA 22 is shown by the above-mentioned expression (1).
The permissible maximum value PSM (dBm) of the average light power in the assumed range of wavelengths is obtained when the light of wavelength xcexH that gives the maximum gain GsH (dB) of the SOA 22 enters, and it is expressed as below.
PSIM(dBm)=PSM(dBm)xe2x88x92GsH(dB)xe2x80x83xe2x80x83(7)
Therefore, on the input side of the SOA 22, the average light input power of the SOA 22 is adjusted to be PSIM (dBm) regardless of the wavelength, as described in FIGS. 1 and 2. In other words, the average light output power PSM1 (dBm) of the SOA 22 when the light of wavelength xcexH enters becomes PSM (dBm).
On the contrary, the average light output power of the SOA 22, when the light of wavelength xcexL that gives the minimum gain GsL (dB) of the SOA 22 in the assumed range of wavelengths is entered, becomes the minimum value PSM2 (dBm) and expressed as below.
PSM2(dBm)=PSIM(dBm)+GsL(dB)xe2x80x83xe2x80x83(8)
Because it is preferable that the output of the light amplifying device 21 is as large as possible, when the average light output power of the SOA 22 is PSM2 (dBm), the quantity of attenuation of the second attenuator 27 is regarded as the basic loss LA2 of the second attenuator 27. Moreover, taking the loss LD2 of the second divider 28 into account, the minimum value of the average light output power of the light amplifying device, that is the output of the second divider 28, is obtained by subtracting LA2 and LD2 from PSM2 (dBm). Therefore, the target value POSET when controlling the output of the light amplifying device 21 is set to this value. That is,
POSET(dBm)=PSM2(dBm)xe2x88x92LA2(dB)xe2x88x92LD2(dB) =PSIM(dBm)+GsL(dB)xe2x88x92LA2(dB)xe2x88x92LD2(dB) =PSM(dBm)xe2x88x92[(GSH(dB)xe2x88x92GsL(dB)) +LA2(dB)+LD2(dB)]xe2x80x83xe2x80x83(9)
On the other hand, the lower limit PIMIN of the average light input power of the light amplifying device is,
PIMIN (dBm)=PSIM(dBm)+LA1+LD1,
as mentioned above, and the upper limit PIMAX is,
PIMAX (dBm)=PIMIN (dBm)+xcex94PIN(dB) =PSIM(dBm)+LA1+LD1+LATM(dB),
as mentioned above.
When used in the wavelength division multiplexed communication system, it is necessary to use a light amplifying device having the ALC function as shown in FIG. 3, but two systems to control the light power are needed on both input and output sides and this leads to a larger scale of the device and, accordingly, a higher cost.
A problem in that the set value of the average light output power of the light amplifying device becomes smaller than the maximum average light output power of the SOA when the wavelength is fixed by [(GSH (dB)xe2x88x92GsL (dB))+LA2 (dB)+LD2 (dB)], as shown in the expression (9) is brought about.
The object of the present invention is to realize a light amplifying device which can be used in a wide range of wavelengths with a comparably simple structure and employs a SOA that can carry out the automatic control of the light output level at a high power level.
To realize the above-mentioned object, the gain of a semiconductor optical amplifier (SOA) is adjusted by the injection of light into the SOA in the present invention. FIG. 5 shows the basic structure of a light amplifying device 31 of the present invention. As shown schematically, in the light amplifying device of the present invention, the signal light input into a combiner 33 and the continuous wave (CW) output from a control light source 38 are combined and entered into a SOA 32. The control light is removed from the light emitted from the SOA 32 by a filter 34, the signal light after amplified in a divider 35 is divided, and the power of one of the divided light is detected by a power meter 36. A control unit 37 changes the power of the control light output from the control light source 38 according to the power detected by the power meter 36. By changing the power of the control light to be input into the SOA 32, and therefore, by changing the consumed amount of the carrier in the active layer due to the induced stimulated emission process in the SOA 32, the gain is adjusted. In this structure, the power of the control light is increased and the carrier in the active layer density is decreased to reduce the gain, but as disclosed in the document A (IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 8, NO 1, JANUARY 1996 (Improvement of Saturation Output Power in a Semiconductor Laser Amplifier through Pumping Light Injection) (Manabu Yoshino and Kyo Inoue), the carrier life is reduced due to the induced stimulated emission process, and the saturation light output power of the SOA 32 is increased. That is, it is possible to reduce gain while increasing the saturation light output power.
It is also possible to adjust the variations of the output light power due to the wavelength dependence of the SOA gain by adjusting the control light power, therefore, the average light output power of the SOA can be set to PSM1 (dBm) regardless of the wavelength. The value that is obtained by subtracting the basic loss of the filter 34, LF (dBm), and the loss of the divider 35, that is LD2 (dB), from PSM1 becomes the average light output power of the light amplifying 31, that is PO (dBm), and shown by the following expression,
xe2x80x83PO(dBm)=PSM1(dBm)xe2x88x92LF(dB)xe2x88x92LD2(dB)
The set value of the average light output power of the light amplifying device, that is POSET (dBm) is assumed to the above-mentioned PO (dBm), when the level of the output signal light of the light amplifying device 31 is controlled to a fixed value. Therefore,
POSET(dBm)=PO(dBm) =PSM1(dBm)xe2x88x92LF(dB)xe2x88x92LD2(dB).
The output light level thus can be controlled to a fixed value at a power higher than that in the conventional structure in FIG. 3 by (GsH (dB)xe2x88x92GsL (dB)+LA2xe2x88x92LF).
As described above, the saturated light output is increased when the control light is injected into the SOA, therefore, it is possible to widen the input dynamic range upward because the permissible maximum light input power increases. It is also possible to apply to the wavelength division multiplex communication system while realizing a constant control of the output light level at a high power in a comparatively simple structure, because the variations of the output light power due to the wavelength dependence of the SOA gain can be controlled by adjusting the control light power.